If anyone is having difficulties sending e-mails out from a Cogeco e-mail, I received a reply from them posted below. Iíve talked to a couple people that have had issues recently (including myself) so I contacted Cogeco and would suggest you do the same if you too are having problems.

The following message was undeliverable.
The reason for the problem:
5.1.0 - Unknown address error 550-"This system is configured to reject mail from 216.221.81.30 [216.221.81.30] (Host blacklisted - Found on Realtime Black List server 'bl.spameatingmonkey.net')"

Response:

Thank you for contacting Cogeco Technical Support regarding your Internet service issues. We are currently experiencing a problem where some customers are getting a bounce back message from certain ISP's who subscribe to SORBS or RBL(real time blacklists) domains.

Unfortunately, we are working diligently to get the Cogeco servers removed from these lists but it does take time. We would suggest that if you have another email address such as "hotmail or gmail", that you use them for important outbound email that needs to get through.